Electrical systems commonly require measurement of resistances or of other parameters associated with resistors or other components within a circuit. In thick or thin film circuit fabrication, for example, arrays of interconnected resistances are monitored, one by one, to determine whether the resistance values are within tolerance limits or to control trimming. Test voltages are applied selectively to the components to be tested through a bank of relay contacts, preferably within frame-type relays, but alternatively within reed, solid state or other type relays. Selective closing of the relay contacts is controlled by a central controller, depending upon the particular component under test and the polarity of voltage to be applied across that component.
Leakage resistance inherent across the terminals of each pair of open contacts is extremely high, on the order of hundreds of megohms. These leakage resistances, despite being extremely high in magnitude, tend to add a significant amount of error to high impedance measurements, e.g., on the order of tens of megohms and higher, particularly when several switches are connected simultaneously to the component being measured.
A principal object of the invention, therefore, is to eliminate the effect of inter-contact leakage in switches.
Another object is to provide a component scanner using an array of switches wherein the effects of leakage resistances within the switches are eliminated.
A further object of the invention is to provide, in a component scanner or other circuit involving signal path switching, at least one switch that is "guarded" by the input signal to eliminate effects of leakage resistance within the switch.
An additional object is more generally to improve the accuracy of high impedance measurements by eliminating the effect of open circuit leakage resistances in measurement circuitry.